To cook fajitas on a propane grill, begin by preheating the grill to medium-high heat. While the grill is heating up, prepare your fajita ingredients.Start by slicing 1-2 pounds of chicken, beef, or shrimp into thin strips. Julienne one large onion and bell pepper, and mince a few cloves of garlic. Once the ingredients are prepped, you can move onto marinating.Prepare a marinade by combining lime juice, olive oil, minced garlic, chili powder, cumin, salt, and pepper in a bowl.